As known, in the paper industry a variety of types of machines and of processes are used for making handkerchiefs, paper towels and the like in packages of interfolded sheets of a certain stack height.
They are obtained stacking the sheets in an “interfolded” way, i.e. at each fold a wing of the previous sheet and a wing of the next sheet engage with each other. This way, when drawing a sheet from the package, at the moment of the use also a wing of the next sheet protrudes, with subsequent practical employment for certain types of users. Among the possible interfolding ways the L, Z or W interfolded sheets are known respectively with 2, 3 and 4 folds per sheet.
A particularly critical step of the process of production of packages of interfolded sheets is the separation of a stack from the next one once achieved a determined height of the pack. In paper industry, in fact, high speed working is necessary during the whole production process and the step of separation of successive stacks of sheets represents a critical point of the whole process since it needs a succession of operations that unavoidably slows down productivity.
For this reason solutions are looked for to provide the high speed separation of the stacks.
The different existing apparatus have all a couple of folding rollers that supply the interfolded sheets on a table and, in some cases, a plurality of “folding arms” that go in and out special grooves made in the folding rollers, thus forming a stack of interfolded sheets on the table.
In U.S. Pat. No. 4,770,402, when the stack of sheets present on the table reaches a certain height, a first series of means of separation moves into the stack from a side whereas a second plurality moves into the stack from the opposite side, and both for all its width. This way, a pack is separated from the next one and precisely the pack of predetermined height is arranged between the table and the first series of means of separation, whereas the other series of means of separation supports the pack being formed up to the moment when the table comes back. The latter, in fact, leaves the pack with the predetermined height on a conveyor belt and moves back in the stack receiving position for receiving the pack.
These machines have, however, different drawbacks which are caused by the shape and the way of operating the means of separation.
The means of separation as above described, in fact, all rotate about an axis for their introduction and the withdrawal into/from the stack being formed. Furthermore, to assure an effective separating action, owing to the width of the means of separation, it is necessary that the latter are inserted into the stack in a high position, near the point of contact of the folding rollers. Therefore, it is necessary that the folding rollers are subject to an additional machining for making the necessary boundary grooves with subsequent additional costs for the machine. Furthermore, the presence of such grooves, making a structural irregularity, limits substantially the quality of the final product and the maximum speed of the machine.
A solution for avoiding to make the boundary grooves in the folding rollers is described in U.S. Pat. No. 5,730,695, where the means of separation enter into the stack only for a portion of its width and from both sides, for allowing a wing of sheet to protrude downwards. At the moment of separation, furthermore, a sheet is withdrawn, and then inserted again into the formed stack, after the next separation.
Also the table is provided in two halves capable of opening for allowing to a wing of sheet to hang towards below. However, the fact of withdrawing a sheet from the stack and insert it again into the formed stack is a drawback. This has the consequence that, in fact, at the moment of the use a sheet is wasted, since it exits double from the pack. Or, that sheet remains on the bottom of the box, since it is difficult to pick up.